1. Field of the Invention
The present invention relates to a zoom optical system and an image pickup apparatus using the same and, more particularly, to an optical arrangement which is suitable for use in a video camera, a still video camera, a copying machine or the like.
2. Description of Related Art
Various photographing optical systems which utilize reflecting surfaces such as concave mirrors and convex mirrors have heretofore been proposed. FIG. 1 is a schematic view of a so-called mirror optical system which is composed of one concave mirror and one convex mirror.
In the mirror optical system shown in FIG. 1, a light beam 104 from an object is reflected by a concave mirror 101 and travels toward an object side while being converged, and after having been reflected by a convex mirror 102, the light beam 104 forms an image of the object on an image plane 103.
This mirror optical system is based on the construction of a so-called Cassegrainian reflecting telescope, and is intended to reduce the entire length of the optical system by folding, by using two opposed reflecting mirrors, the optical path of a telephoto lens system which is composed of refracting lenses and has an entire large length.
For similar reasons, in the field of an objective lens system which constitutes part of a telescope lens system as well, a multiplicity of types which are arranged to reduce the entire length of an optical system by using a plurality of reflecting mirrors have been proposed.
It has heretofore been known that there are methods for obtaining a compact mirror optical system by using reflecting mirrors in place of a lens optical system having an entire large length to efficiently fold an optical path in the above-described manner.
However, in general, the mirror optical system, such as the Cassegrainian reflecting telescope, has the problem that part of an object ray is blocked by the convex mirror 102.
To solve the problem, it has been proposed to provide a mirror optical system which uses decentered reflecting mirrors to prevent a member of the optical system from blocking the area through which the object light beam 104 passes, i.e., to separate a central ray 106 of the object light beam 104 from an optical axis 105.
FIG. 2 is a schematic view of the mirror optical system disclosed in U.S. Pat. No. 3,674,334. This mirror optical system solves the above-described blocking problem by separating a central ray 116 of an object light beam 115 from an optical axis 114 by decentering the central axis of reflecting mirrors from the optical axis 114.
In the mirror optical system shown in FIG. 2, a concave mirror 111, a convex mirror 113 and a concave mirror 112 are arranged in the order of passage of the light beam, and these mirrors 111, 113 and 112 are reflecting mirrors which are rotationally symmetrical about the optical axis 114, as shown by two-dot chain lines in FIG. 2. In the shown mirror optical system, a central ray 116 of an object light beam 115 is separated from the optical axis 114 to prevent blockage of the object light beam 115, by using only the upper portion of the concave mirror 111 which is above the optical axis 114 as viewed in FIG. 2, only the lower portion of the convex mirror 113 which is below the optical axis 114 as viewed in FIG. 2, and only the lower portion of the concave mirror 112 which is below the optical axis 114 as viewed in FIG. 2.
FIG. 3 is a schematic view of the mirror optical system disclosed in U.S. Pat. No. 5,063,586. The mirror optical system shown in FIG. 3 solves the above-described problem by decentering the central axis of each reflecting mirror from an optical axis and separating the principal ray of an object light beam from the optical axis.
As shown in FIG. 3 in which an axis perpendicular to an object plane 121 is defined as an optical axis 127, a convex mirror 122, a concave mirror 123, a convex mirror 124 and a concave mirror 125 are arranged in the order of passage of the light beam, and the central coordinates and central axes 122a, 123a, 124a and 125a (axes which respectively connect the centers of reflecting surfaces and the centers of curvature thereof) of the reflecting surfaces of the respective mirrors 122 to 125 are decentered from the optical axis 127. In the mirror optical system shown in FIG. 3, by appropriately setting the amount of decentering and the radius of curvature of each of the surfaces, each of the reflecting mirrors is prevented from blocking an object light beam 128, so that an object image is efficiently formed on an image forming plane 126.
In addition, U.S. Pat. Nos. 4,737,021 and 4,265,510 also disclose an arrangement for preventing the blocking problem by using part of a reflecting mirror which is rotationally symmetrical about an optical axis, or an arrangement for preventing the blocking problem by decentering the central axis of the reflecting mirror from the optical axis.
In addition, a zooming art is known which varies the image forming magnification (focal length) of a photographing optical system by relatively moving a plurality of reflecting mirrors which constitute part of the aforesaid type of mirror optical system.
For example, U.S. Pat. No. 4,812,030 discloses an art for performing variation of the magnification of the photographing optical system by relatively varying the distance between the concave mirror 101 and the convex mirror 102 and the distance between the convex mirror 102 and the image plane 103 in the construction of the Cassegrainian reflecting telescope shown in FIG. 1.
FIG. 4 is a schematic view of another embodiment disclosed in U.S. Pat. No. 4,812,030. In the embodiment shown in FIG. 4, an object light beam 138 from an object is made incident on and reflected by a first concave mirror 131, and travels toward an object side as a converging light beam and is made incident on a first convex mirror 132. The light beam is reflected toward an image forming plane by the first convex mirror 132 and is made incident on a second convex mirror 134 as an approximately parallel light beam. The light beam is reflected by the second convex mirror 134 and is made incident on a second concave mirror 135 as a diverging light beam. The light beam is reflected by the second concave mirror 135 as a converging light beam and forms an image of the object on an image plane 137.
In this arrangement, a distance 133 between the first concave mirror 131 and the first convex mirror 132 and a distance 136 between the second convex mirror 134 and the second concave mirror 135 are varied to perform zooming, thereby varying the focal length of the entire system.
In the arrangement disclosed in U.S. Pat. No. 4,993,818, an image formed by the Cassegrainian reflecting telescope shown in FIG. 1 is secondarily formed by another mirror optical system provided in a rear stage, and the magnification of the entire photographing optical system is varied by varying the image forming magnification of that secondary image forming mirror optical system.
In any of the above-described reflecting types of photographing optical systems, a large number of constituent components are needed and individual optical components need to be assembled with high accuracy to obtain the required optical performance. Particularly in the type in which decentered reflecting mirrors constitute a mirror optical system as in the conventional example shown in FIG. 3, the reflecting mirrors must be individually disposed with different amounts of decentering, so that an extremely high assembling accuracy is needed.
One proposed approach to solving this problem is to eliminate the incorporation error of optical components which occurs during assembly, as by forming a mirror system as one block.
A conventional example in which a multiplicity of reflecting surfaces are formed as one block is an optical prism, such as a pentagonal roof prism and a Porro prism, which is used in, for example, a viewfinder optical system.
In the case of such a prism, since a plurality of reflecting surfaces are integrally formed, the relative positional relationships between the respective reflecting surfaces are set with high accuracy, so that adjustment of the relative positions between the respective reflecting surfaces is not needed. Incidentally, the primary function of the prism is to reverse an image by varying the direction in which a ray travels, and each of the reflecting surfaces consists of a plane surface.
Another type of optical system, such as a prism having reflecting surfaces with curvatures, is also known.
FIG. 5 is a schematic view of the essential portion of the observing optical system which is disclosed in U.S. Pat. No. 4,775,217. This observing optical system is an optical system which not only allows an observer to observe a scene of the outside but also allows the observer to observe a display image displayed on an information display part, in the form of an image which overlaps the scene.
In this observing optical system, a display light beam 145 which exits from the display image displayed on an information display part 141 enters from an entrance surface 148, and is reflected by a surface 142 and travels toward an object side and is made incident on a half-mirror surface 143 consisting of a concave surface. After having been reflected by the half-mirror surface 143, the display light beam 145 is formed into an approximately parallel light beam by the refractive power of the half-mirror surface 143. This approximately parallel light beam is refracted by and passes through a surface 142, and forms a magnified virtual image of the display image and enters a pupil 144 of an observer so that the observer recognizes the display image.
In the meantime, a light beam 146 from the outside is made incident on a surface 147 which is approximately parallel to the reflecting surface 142, and is then refracted by the surface 147 and reaches the half-mirror surface 143 which is a concave surface. Since the half-mirror surface 143 is coated with an evaporated semi-transparent film, part of the light beam 146 passes through the half-mirror surface 143, is refracted by and passes through the surface 142, and enters the pupil 144 of the observer. Thus, the observer can visually recognize the display image as an image which overlaps the scene of the outside.
FIG. 6 is a schematic view of the essential portion of the observing optical system disclosed in Japanese Laid-Open Patent Application No. Hei 2-297516. This observing optical system is also an optical system which not only allows an observer to observe a scene of the outside but also allows the observer to observe a display image displayed on an information display part, as an image which overlaps the scene.
In this observing optical system, a display light beam 154 which exits from a display image displayed on an information display part 150 enters a prism Pa through a plane surface 157 which constitutes part of the prism Pa, and is made incident on a parabolic reflecting surface 151.
The display light beam 154 is reflected by the reflecting surface 151 as a converging light beam, and forms an image on a focal plane 156. At this time, the display light beam 154 reflected by the reflecting surface 151 reaches the focal plane 156 while being totally reflected between two parallel plane surfaces 157 and 158 which constitute part of the prism Pa. Thus, the thinning of the entire optical system is achieved.
Then, the display light beam 154 which exits from the focal plane 156 as a diverging light beam is totally reflected between the plane surface 157 and the plane surface 158, and is made incident on a half-mirror surface 152 which consists of a parabolic surface. The display light beam 154 is reflected by the half-mirror surface 152 and, at the same time, not only is a magnified virtual image of a display image formed but also the display light beam 154 is formed into an approximately parallel light beam by the refractive power of the half-mirror surface 152. The obtained light beam passes through the surface 157 and enters a pupil 153 of the observer, so that the observer can recognize the display image.
In the meantime, an object light beam 155 from the outside passes through a surface 158b which constitutes part of a prism Pb, then through the half-mirror surface 152 which consists of a parabolic surface, then through the surface 157, and is then made incident on the pupil 153 of the observer. Thus, the observer visually recognizes the display image as an image which overlaps the scene of the outside.
As another example which uses an optical unit on a reflecting surface of a prism, optical heads for optical pickups are disclosed in, for example, Japanese Laid-Open Patent Application Nos. Hei 5-12704 and Hei 6-139612. In these optical heads, after the light outputted from a semiconductor laser has been reflected by a Fresnel surface or a hologram surface, the reflected light is focused on a surface of a disk and the light reflected from the disk is conducted to a detector.
In any of the above-described mirror optical systems having the decentered mirrors, which are disclosed in U.S. Pat. Nos. 3,674,334, 5,063,586 and 4,265,510, since the individual reflecting mirrors are disposed with different amounts of decentering, the mounting structure of each of the reflecting mirrors is very complicated and the mounting accuracy of the reflecting mirrors is very difficult to ensure.
In either of the above-described photographing optical systems having the magnification varying functions, which are disclosed in U.S. Pat. Nos. 4,812,030 and 4,993,818, since a large number of constituent components, such as a reflecting mirror or an image forming lens, are needed, it is necessary to assemble each optical part with high accuracy to realize the required optical performance.
In particular, since the relative position accuracy of the reflecting mirrors is strict, it is necessary to adjust the position and the angle of each of the reflecting mirrors.
As is known, conventional reflecting types of photographing optical systems have constructions which are suited to a so-called telephoto lens using an optical system having an entire large length and a small angle of view. However, if a photographing optical system which needs angles of view from an angle of view of a standard lens to an angle of view of a wide-angle lens is to be obtained, the number of reflecting surfaces which are required for aberration correction must be increased, so that a far higher component accuracy and assembly accuracy are needed and the cost and the entire size of the optical system tend to increase.
Either of the observing optical systems disclosed in U.S. Pat. No. 4,775,217 and Japanese Laid-Open Pat. Application No. Hei 2-297516 is primarily intended to vary the direction of travel of a ray and a pupil's image forming action for efficiently transmitting to the pupil of an observer a display image displayed on the information display part which is disposed away from the pupil of the observer. However, neither of them directly discloses an art for performing positive aberration correction by using a reflecting surface having a curvature.
The range of applications of either of the optical systems for optical pickups which are disclosed in, for example, Japanese Laid-Open Pat. Application Nos. Hei 5-12704 and Hei 6-139612 is limited to the field of a detecting optical system, and neither of them satisfies the image forming performance required for, particularly, an image pickup apparatus which uses an area type of image pickup element, such as a CCD.